1. Field of the Invention
This invention relates to coke and more particularly refers to a new and improved method for the manufacture of coal-tar pitch with a predetermined degree of anisotropy and coefficient of volume expansion.
2. Description of the Prior Art
From German Published Prosecuted application 1 189 517 it was known to manufacture coal-tar pitch coke with a low thermal coefficient of expansion and needle-like texture from coal tar pitches called tars from which "soot-like" substances have been separated. By the term "soot-like" substances are meant substances insoluble in quinoline which in addition to soot and minerals are high-molecular, resinous compounds, mostly aromatic in nature. These substances, which are insoluble in quinoline are separated by separators, centrifuges or filters from tar which has been treated with suitable solvents as for example tar oils, and heated above the temperature of the softening point. The tar which has been freed from soot, minerals and high-molecular aromatic compounds, is pyrolized in ovens such as are usually used for producing normal coal-tar pitch coke, by special multi-step carbonizing processes or by a delayed coking process to produce a coal-tar pitch coke with a thermal volume expansion coefficient of less than 6 .times. 10.sup.-6 /K.
Vectorial properties of these cokes, such as the linear thermal expansion coefficient, electrical resistance, strength, and others are to a great extent dependent on the spatial direction in which they are observed. This dependence of direction, called anisotropy, is usually numerically defined as anisotropy-ratio or degree of anisotropy and, for example, the degree of anisotropy of the linear thermal expansion coefficients is approximately 1.6 to 2.0. Graphite bodies made from these cokes also show a relatively low volume expansion coefficients and a high degree of anisotropy. Because of their low electrical resistance in the axial direction and their outstanding stability against rapid temperature changes, these graphite bodies are particularly well suited for the production of electro-steel. Graphites made from cokes with a high degree of anisotropy are less suitable for a number of other uses, for example structural parts or parts for moderators for high temperature reactors, since the changes in length caused by the neutron radiation are also a function of the spatial direction, causing the original shape of the bodies to change during radiation or the accumulation of stresses from such radiation can lead to the formation of cracks in the graphite. Cokes with a low degree of anisotropy (isotropic or quasi-isotropic cokes) are preferred for this type of application.
Several methods have become known for the production of an isotropic coke by the pyrolization of coal-tar pitch or derivatives of tars. For example in the method according to the German Published Non-Prosecuted application 2 300 023, the tar distillate in the temperature range of 250 to 420.degree. C., before pyrolization, treated, i.e. blown with a gas mixture containing elementary oxygen. The coal-tar pitch cokes that result from a process of this type are nearly isotropic -- the degree of anisotropy of the linear expansion coefficient is 1.2 or less -- and are suitable for the manufacture of graphite for high-temperature reactors. These cokes, however, show a thermal volume expansion coefficient larger than about 15 .times. 10.sup.-6 /K and therefore are not suitable as starting material for production of graphite bodies which are, for example, subject to higher temperature requirements.